Let's academically disclaim that the book "Mormon Doctrine" is not actually a book of mormon doctrine, nor is it published or sold by said church. It's the writings of a man, who had his own opinions. Granted, he was a church leader with a strong academic background, but nonetheless he was speaking outside of his calling on topics the church didn't have a statement on (like the evils of psychiatry and tattoos
Anyway, I just write that to say that the quote above may or may not have been respresentative of a statistically significant proportion of members of the Mormon church. I simply don't know. Most historians will tell you Joseph Smith's strict abolitionist arguments were a chunk of what got him killed... at the same time, all churches as far as I'm aware accepted the Cain/dark-skin argument, because it's a common reading of the Book of Genesis. Said argument has nothing to do with current Mormonism, however.